Ultrasound flow measuring devices are often used in process and automation technology. They permit volume flow and/or mass flow in a pipeline to be determined in simple manner.
Known ultrasound flow measuring devices frequently work according to the Doppler principle or according to the travel time difference principle.
In the case of the travel time difference principle, the different travel times of ultrasonic pulses are evaluated relative to the flow direction of the liquid.
For this, ultrasonic pulses are transmitted at a certain angle to the tube axis, both in the direction of flow as well as also counter to the flow. From the travel time difference, the flow velocity can be determined, and therewith, in the case of a known diameter of the pipeline section, also the volume flow.
In the case of the Doppler principle, ultrasonic waves with a certain frequency are coupled into the liquid and the ultrasonic waves reflected by the liquid are evaluated. From the frequency shift between the in-coupled and the reflected waves, the flow velocity of the liquid can likewise be determined.
Reflections in the liquid occur, however, only when small air bubbles or impurities are present in the liquid, so the Doppler principle is made use of mainly in the case of contaminated liquids.
The ultrasonic waves are produced or received with the assistance of so-called ultrasonic transducers. For this, ultrasonic transducers are firmly attached to the tube wall of the relevant pipeline section. More recently, clamp-on ultrasound flow measuring systems are also obtainable. In the case of these systems, the ultrasonic transducers are only pressed onto the tube wall with a clamp. Such systems are known, for example, from EP 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
A further ultrasound flow measuring device, which works according to the travel time difference principle, is known from U.S. Pat. No. 5,052,230. The travel time is ascertained here by means of short ultrasonic pulses.
A large advantage of clamp-on ultrasound flow measuring systems is that they do not contact the measured medium and are placed on an already existing pipeline.
The ultrasonic transducers are normally composed of a piezoelectric element, also called a “piezo” for short, and a coupling layer, also called a “coupling wedge” or, less frequently, a “lead-in element”. The coupling layer is, in such case, most often manufactured from synthetic material; the piezoelectric element is, in industrial process measurements technology, usually composed of a piezoceramic. In the piezoelectric element, the ultrasonic waves are produced, and, via the coupling layer, are conducted to the tube wall, and are from there led into the liquid. Since the velocities of sound in liquids and synthetic materials are different, the ultrasonic waves are refracted during the transition from one medium to the other. The angle of refraction is determined to a first approximation according to Snell's law. The angle of refraction is thus dependent on the ratio of the propagation velocities in the media.
Between the piezoelectric element and the coupling layer, another coupling layer can be arranged, a so-called adapting or matching layer. In such case, the adapting or matching layer performs the function of transmission of the ultrasonic signal, and simultaneously of reduction of a reflection off of interfaces between two materials caused by different acoustic impedances.
In U.S. Pat. No. 5,179,862, an ultrasonic measuring system is disclosed, wherein a flexible measuring tube—a hose—can be drawn into in a fixed apparatus. The fixed part of the measuring system accommodates the ultrasonic transducer. If an ultrasonic signal runs axially through the measuring tube, the measuring tube is bent at two locations lying opposite each other. Between the bends is thus located the actual measuring path of the measuring tube; the bends could be considered as a measuring tube inlet and measuring tube outlet with the same diameter as the measuring tube. At the bends, the coupling elements of the ultrasonic transducers couple the ultrasonic signals into the measuring tube and, respectively, out of the measuring tube. In this regard, the coupling elements have corresponding in-coupling and/or out-coupling surfaces, which, in each case, form an angle with respect to the piezoelectric elements, wherein the piezoelectric elements, for their own part, are arranged planparallel to one another and perpendicular to the measuring tube axis. The sound-emitting or sound-receiving surfaces of the piezoelectric elements are at least the size of the diameter of the measuring tube. A disadvantage of angles between the sound-emitting or sound-receiving surfaces of the piezoelectric elements and the in-coupling or out-coupling surfaces of the coupling elements is that the velocities of sound in the coupling elements and in the measuring tube and/or in the measured medium must be matched to one another and with the angles to obtain a signal path for the ultrasonic signal, which is parallel to the measuring tube axis.
Other examples of ultrasound flow measuring systems with ultrasonic transducers arrangeable axially on the ends of a measuring tube are shown in U.S. Pat. No. 5,463,906 and U.S. Pat. No. 5,717,145. Also applied are piezoelectric elements with sound-emitting or sound-receiving surfaces, which at least correspond to the size of the diameter of the measuring tube, in order to metrologically register the entire measuring tube cross section.
Also EP 1 760 436 A2 shows an ultrasound flow measuring system with a measuring tube, through which ultrasound waves are axially projected. The measuring tube includes piezoelectric elements, whose sound-emitting or sound-receiving surfaces have a diameter, which is at least as large as the diameter of the measuring tube.
In contrast, in US 2007/0227263 A1, an ultrasound flow measuring system is described, which has a measuring tube, which has four openings. Two openings are provided for the connection to the process. These openings form, respectively, a measuring tube inlet, and a measuring tube outlet, and which are perpendicular to the measuring tube axis. The measuring tube inlet and the measuring tube outlet are both on the same side of the measuring tube. The ultrasonic transducers are inserted in the two other openings of the measuring tube. The coupling elements of the ultrasonic transducers axially close off the measuring tube. The ultrasonic transducers are arranged essentially planparallely and at an angle of 90° with respect to the measuring tube axis. The in-coupling and/or out-coupling surfaces of the coupling elements of the ultrasonic transducer for coupling the ultrasonic signals into and out of the measured medium are convex. Directly before the in-coupling and/or out-coupling surfaces, thus in the region of the coupling of the ultrasonic signals into or out of the measured medium, the measuring tube has an enlarged cross section. The diameters of the essentially disc-shaped ultrasonic transducers are larger than the diameters of a significant part of measuring path of the measuring tube. Due to the cross sectional change of the measuring tube before the in-coupling and/or out-coupling surfaces, there arise chambers, where the measured medium can collect just before the in-coupling and/or out-coupling surfaces. Since the diameter of the measuring tube inlets or measuring tube outlets and the measuring path of the measuring tube itself are essentially equal, the flow of the measured medium before the in-coupling and/or out-coupling surfaces is slowed, and in the measuring tube, in an essential part the measuring path, it is accelerated.
WO 2008/101662 A2 also has a similar construction of the measuring cell. Here, the diameters of the measuring tube inlets or measuring tube outlets and of the measuring tube itself are essentially equal, and the measuring tube inlets or measuring tube outlets are arranged at an angle smaller than 90° relative to the measuring tube axis; however, the flow of the measured medium is also slowed here before the in-coupling and/or out-coupling surfaces, which here are also in turn formed by the coupling elements of the ultrasonic transducers installed into the measuring cell, and are arranged at an angle of 90° to the measuring tube axis. Here, chambers are likewise formed in front of the in-coupling and out-coupling surfaces. The sound-emitting and sound-receiving surfaces of the piezoelectric elements are each larger than the diameter of the measuring tube; however, the surface of the coupling element for coupling sound in or out axially closes off the measuring tube and is, consequently, of equal size.